Andros Barrier Reef

• Category: Marine and Oceanic Biomes.
• Geographic Location: North Atlantic.
• Summary: The Andros Barrier Reef, one of the world's largest coral reef complexes, is a valuable habitat for many species in the western North Atlantic Ocean.

Andros, the largest island of the Bahamas, possesses a reef complex lying along its entire eastern coastline. The reef is a somewhat discontinuous structure that extends for approximately 135 miles (217 kilometers) from Joulter Cays southward to South Cay. This reef complex is the second-largest in the western Atlantic and considered to be the third-largest reef in the world.

Although the Andros reef is sometimes referred to as a barrier reef, its structure and origins indicate that it is not truly a barrier reef in the strictest sense. Barrier reefs, as described by Charles Darwin, are formed over long periods of time by the simultaneous growth of shallow-water corals along a tropical coastline and the slow subsidence of that coastline and the adjacent land mass. The resulting reef lies many miles offshore, with a deep lagoon separating the reef from the adjacent land.

By contrast, the Andros reef complex parallels the island coastline but is separated from it by a relatively narrow and shallow lagoon. The reef likely originated from corals growing along the seaward edge of a newly submerged rocky platform that was created during a time of lower sea levels. Such reefs are common in the Caribbean and are considered to be hybrid structures that are sometimes referred to as bank-barrier reefs.

Reef Habitats

The predominant habitats along the reef are the shallow reef crest and the deeper fore reef that extends toward the open ocean. Occasional patch reefs also occur behind the reef crest in the lagoon. The reef crest is dominated by colonies of Elkhorn coral (Acropora palmata). The thick branches of Elkhorn coral are ideally suited to withstand the breaking waves common on the reef crest. Star corals (Montastrea spp.) predominate in the fore reef zone. These colonies form boulder-like structures or may be somewhat flattened at increasing depths. The region known as the Tongue of the Ocean lies a short distance seaward from the fore reef. Here, the ocean floor drops precipitously to depths of approximately 6,000 feet (1,829 meters).

Fish Species

More than 160 species of fish inhabit the Andros reef complex. Generally, the reef crest possesses higher densities of fish than the fore reef. The fish community is dominated by species of grunts, parrotfish, snappers, tangs, and surgeonfish, as well as large schools of smaller fishes such as silversides, herrings, and anchovies.

Groupers are also found along the length of the reef, but in lower numbers than other species of fish. Groupers are major predators on the reef and are currently under stress throughout the area due to commercial fishing in the Caribbean. The Andros reef is especially noted for the large spawning aggregations of Nassau grouper that occur in the central and southern portions of the complex. Although solitary for most of their lives, groupers may travel each year from as far as 100 miles (161 kilometers) away to arrive at the reef for their mass spawning events, which occur during the full moon in December and January. Spawning aggregations vary in size but can include thousands of fish. As the sun sets, mates gather in smaller subgroups within the aggregation, and the release of gametes occurs. Offspring are transported across the reef and into the protected waters of the lagoon. Young groupers use the quiet seagrass beds within the lagoon to feed and grow before returning to the reefs and rocky bottom areas inhabited by their parents.

Human Activities

Fishing pressure along the reef is considered to be light to moderate compared with other regions of the western North Atlantic and the Caribbean. Although groupers and snappers are the primary fishes targeted, both commercial and subsistence fisherman will also take grunts, hogfish, triggerfish, and barracuda. Groupers are particularly vulnerable to capture during spawning aggregations, and their protection during that time of year is important to the sustainability of the fishery. Andros Island is sparsely populated, and local land uses do not appear to greatly affect the reef.

Ecosystem Stresses

Regional and global stresses to the reef have been documented. A lack of herbivorous fish and invertebrates, for example, has likely allowed the overgrowth of algal turfs on some parts of the reef. These turfs cover rocks and rubble and can reduce sites where new coral colonies can become established. This is particularly true of the fore reef, which does not exhibit as many herbivorous fish species as the reef crest.

Populations of another important herbivore, the long black-spined sea urchin (Diadema antillarum), diminished severely during a Caribbean-wide disease outbreak in the mid-1980s. This decline has resulted in a predominance of algal turfs over many reefs in the region. Urchin populations are slowly beginning to recover on the Andros reef but have not yet reached sufficient sizes to act as effective controls for the algae.

Some coral diseases also occur along the Andros reef complex. Evidence of disease is most prevalent in star corals and brain corals on the fore reef. Black-band disease and white plague have both been documented. Black-band disease is a consortium of sulfur-metabolizing bacteria and blue-green algae that invades the coral tissue and leaves bare skeleton in its wake. The pathogen involved in white plague has not been identified, but the results are similar to that of black-band disease, as coral tissue death exposes the colony's skeleton. These conditions can kill or cause partial death of coral colonies. Even if a coral is not completely killed, these diseases can reduce the colony's reproductive ability and open its skeletal structure to bioeroding organisms such as sponges and marine worms.

Finally, coral bleaching, a stress on coral reefs worldwide, has been observed on the Andros reef. Bleaching, which can lead to coral mortality, is the loss of symbiotic algae that are normally found within the coral's tissues. The resident algae impart a color to the colony that varies from yellow to golden-brown to shades of green. Following bleaching, the colony appears to be white, as the white skeleton can be seen through the transparent tissues of the coral. Bleaching is a generalized response to many types of environmental stress but is most often observed on reefs during periods of elevated water temperature. Although no large-scale bleaching events have been documented in Andros, areas of the reef crest are sometimes affected. Despite comparative field research, it is not understood exactly how the Andros has escaped the large-scale bleaching events that have impacted other reefs around the world. Coral colonies can recover from partial bleaching, but even in recovery, the growth and reproductive ability of corals are negatively affected.

Despite the environmental stresses just discussed, the Andros reef is generally considered to be in good condition relative to other reefs in the western North Atlantic and the adjoining Caribbean Sea areas. In 2002, the government of the Bahamas set aside more than 64,000 acres (25,900 hectares) of this reef complex to create two national parks, called the North and South Marine Parks. The establishment of these marine protected areas is an important first step in conserving a beautiful and fragile tropical marine resource.

Bibliography

“The Andros Barrier Reef.” Andros Conservancy and Trust, 2010, . Accessed 10 Oct. 2024.

Casola, William R., et al. “Drivers of Long-Term Support for Marine Protected Areas in The Bahamas.” Ocean and Coastal Management, vol. 217, 2022, doi.org/10.1016/j.ocecoaman.2021.106000. Accessed 10 Oct. 2024.

Kaplan, Eugene. A Field Guide to the Coral Reefs: Caribbean and Florida. Houghton Mifflin, 1982.

Kramer, Philip, et al. “Assessment of Andros Island Reef System, Bahamas, Part I: Stony Corals and Algae.” Atoll Research Bulletin, vol. 496, no. 1, 2003.

Kramer, Philip, et al. “Assessment of Andros Island Reef System, Bahamas, Part II: Fishes.” Atoll Research Bulletin, vol. 496, no. 1, 2003.

“The Nassau Grouper.” Society for the Conservation of Fish Aggregations, .

Sexton, Chrissy. "Andros Island from Space." Earth.com, 18 Apr. 2020, www.earth.com/image/andros-island-from-space/. Accessed 10 Oct. 2024.